Infrared fibre optics for communication and infrared remote control and sensing technology are two important technological areas in modern electronics. In these applications, a sensitive and fast infrared sensor is the most crucial element in the system. Recently, Levine et al have disclosed a new multiple quantum well photodetector. This device is discussed in two separate articles, the first of which is "New 10.mu.m Infrared Detector Using Intersubband Absorption In Resonant Tunneling GaAlAs Superlattices," Applied Physics Letters, Vol. 50 (1987), pg. 1092-1094. The authors introduce a high-speed infrared detector based on intersubband absorption and sequential resonant tunneling in doped GaAs/Al.sub.x Ga.sub.1-x As quantum well superlattices. They term the device a STAIR detector. In this type of detector infrared light which is resonant with the intersubband transition excites electrons from the doped ground state to the excited state where they can tunnel out of the well through the thin top of the barrier. These photogenerated hot electrons then travel a mean free path thereby generating a photocurrent before being captured by the wells "Broadband 8-12.mu.m High-sensitivity GaAs Quantum Well Infrared Photodector," Applied Physics Letters, Vol. 54 (1989), pg. 2704-2706, also by Levine et al contributes no novel features but does extend the range of parameters of the device.
In a related article by Kwong-Kit Choi et al titled "Multiple Quantum Well 10.mu.m GaAs/Al.sub.x Ga.sub.1-x As Infrared Detector With Improved Responsivity," a higher responsivity is achieved in this type of detection device. This occurs by using thicker and higher Al.sub.x Ga.sub.1-x As superlattice barriers to reduce the dark current. This allowed the detector to be operated at higher biases. Additional background can be obtained from European Patent No. 883000 96.0 -"Infrared-Radiation Detector Device," filed by Clyde G. Bethea et al
The essential feature of this type of device is that the detector consists of a number of isolated quantum wells and is capable of detecting radiation of one particular frequency with a certain bandwidth. The detecting frequency cannot be changed by applying voltage bias. However, for optical communication and controlling purposes, it is desirable that the optical sensor can receive parallel coded messages to increase the rate of data transfer and to discriminate faulty signals. In this case, a sensor that is sensitive to more than one frequency is important. The present invention achieves the goal of multicolor infrared detection.